Fetal guinea pig adrenals and gonads are known to produce androgens in vitro (Zaaijer et al, '66; Ortiz et al, '66, '67) and there is biochemical (Bloch, '69) and histochemical (Ortiz et al, '66) evidence that these hormones are steroids. Most steroid-secreting cells have well-developed smooth endoplasmic reticulum and this organelle is a site of major enzymes involved in steroid biosynthesis. Smooth reticulum appears in fetal guinea pig testicular interstitial cells (Black and Christensen, '69) and adrenal cortical cells (Black, '72) at the earliest times the testes and adrenals can be shown to produce androgens in vitro. Extensive development of randomly arranged tubular reticulum occurs in interstitial cells and inner cortical cells, and later complex arrays of smooth reticulum develop. Small membrane-bound granules, resembling peroxisomes are occasionally seen in continuity with the developing reticulum. They increase in number as the cells differentiate. It is the purpose of this study:1) To examine the relationship between the development of smooth reticulum, the development of steroidogenic enzymes, and cholesterol storage utilizing cytochemical and biochemical techniques. 2) To determine if the number or distribution of complex arrays of smooth reticulum can be altered in vivo and in vitro. 3) To examine the role of peroxisomes in steroid-secreting cells during differentiation and function.